Method for improving reliability of copper interconnects

ABSTRACT

Doping copper interconnects ( 100 ) with silicon ( 115 ) has been shown to improve Electromigration and Via Stress Migration reliability. After copper ( 118 ) is deposited by electrochemical deposition and chemically-mechanically polished back, doping is achieved by flowing SiH 4  over the copper interconnect ( 100 ) for 0.5 to 5 seconds at a temperature of 325-425° C.

FIELD OF THE INVENTION

The invention is generally related to the field of forming damascenecopper interconnects in semiconductor devices and more specifically toimproving the reliability of damascene copper interconnects.

BACKGROUND OF THE INVENTION

As the density of semiconductor devices increases, the demands oninterconnect layers for connecting the semiconductor devices to eachother also increases. Therefore, device fabrication technology ismigrating from aluminum metal interconnects to copper interconnects andfrom traditional silicon-dioxide-based dielectrics to low-k dielectrics,such as organo-silicate glass (OSG). Semiconductor fabrication processesthat work with copper interconnects and newer low-k dielectrics arestill being developed and optimized. As compared to the traditionalsubtractive plasma dry etching of aluminum, suitable copper etches for asemiconductor fabrication environment are not readily available. Toovercome the copper etch problem, damascene processes have beendeveloped.

In a damascene process, the IMD (intrametal dielectric) is formed first.The IMD is then patterned and etched to form a trench for theinterconnect line. If connection vias have not already been formed, adual damascene process may be used. In a dual damascene process, viasare etched into the ILD (interlevel dielectric) 12 for connection tolower interconnect levels and trenches are etched into the IMD 14. Thebarrier layer 16 and a copper seed layer are then deposited over openvia/trench structures. The barrier layer 16 is typically tantalumnitride or some other binary transition metal nitride and the thin Cuseed layer is deposited using physical vapor deposition. A copper layeris then electrochemically deposited onto the seed layer that covers theentire structure. The copper is then chemically-mechanically polished(CMP'd) to remove the copper over the IMD 14, leaving copperinterconnect lines 18 and vias 20 as shown in FIG. 1. A metal etch isthereby avoided.

Typically, several copper interconnect layers are successively formed.After one metal interconnect layer is formed, an etchstop layer isdeposited thereover and the next levels' ILD and IMD are formed. Theetchstop layer prevents Cu diffusion from the metal lines into theoverlying oxide-based dielectric and protects the Cu from subsequentvia/trench etches used to form the next level of interconnect.

SUMMARY OF THE INVENTION

The invention is a method for forming a copper interconnect havingimproved reliability. After the copper is plated by electrochemicaldeposition (ECD) and chemically-mechanically polished back, the exposedcopper surface is lightly doped with silicon by flowing a siliconcontaining gas such as SiH₄ over the wafer surface immediately prior todepositing the overlying dielectric layer. Silicon doping improveselectromigration (EM) and via stress migration (VSM) lifetimes byslowing the Cu diffusion rate at the interface between the Cu leads andthe overlying dielectric barrier.

An advantage of the invention is providing a copper interconnect havingimproved reliability using a method that requires no additionalequipment beyond what is normally required to create such interconnects.

This and other advantages will be apparent to those of ordinary skill inthe art having reference to the specification in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-sectional diagram of a conventional (prior art) dualdamascene copper interconnect.

FIG. 2 is a cross-sectional diagram of a copper interconnect formedaccording to an embodiment of the invention.

FIGS. 3A-3E are cross-sectional diagrams of the copper interconnect ofFIG. 2 at various stages of fabrication.

FIG. 4 shows EM lifetime improvement under stress conditions of 325° C.and 1.5 MA/cm² current density for cases of 1) no silane over waferprior to SiN deposition and 2) 3 seconds silane over wafer prior to SiNdeposition.

FIG. 5 is a graph of Via Stress Migration improvement with SiH4 exposureand temperature. Increased SiH4 flow or increased temperature result inincreased Si doping level of the Cu surface.

FIG. 6 is a graph depicting the interface adhesion energy (as determinedby 4-point bending methodology) for the case of various dopingconditions. The interface strength improves with higher doping levels.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will now be described in conjunction with a specific viafirst copper dual damascene process. It will be apparent to those ofordinary skill in the art that the benefits of the invention may beapplied to other copper interconnect processes such as single damasceneprocesses, trench first dual damascene processes and other via firstdual damascene processes.

As the technology scales to below the 130 nm node, the VSM and EMrequirements become more and more difficult to meet. The inventors havediscovered that by doping the copper interconnect with a small amount ofSi after CMP improves the VSM and EM lifetimes. FIG. 4 shows the EMlifetime improvement for a Si-doped vs. undoped interconnect line of˜0.18 μm width and ˜0.35 μm depth under stress conditions of 325° C. and1.5 MA/cm². FIG. 5 shows the improvement in Via Stress Migration 48hr/150 C shift for the case of 0 seconds or 3 seconds SiH₄ flow at 400 Cand 3 seconds SiH₄ flow at 425 C. The EM and VSM improvements areattributed to the improvement in adhesion interface energy between theSiN and the Si-doped Cu, shown in FIG. 6.

A dual damascene copper interconnect 100 formed according to theinvention is shown in FIG. 2. Copper interconnect 100 is formed oversemiconductor body 102. Semiconductor body 102 typically has transistorsand other devices (not shown) formed therein. Semiconductor body 102 mayalso include one or more additional metal interconnect layers (notshown). Copper interconnect 100 comprises a lead portion formed withintrenches 108. A via portion is formed in vias 106 below trenches 108.Vias 106 extend from the bottom of trenches 108 through interleveldielectric (ILD) 110 to a lower metal interconnect layer. Trenches 108are formed within intrametal dielectric (IMD) 112. Various materials areknown to be suitable for forming ILD 110 and IMD 112. For example,fluorine-doped silicate glass (FSG), organo-silicate glass (OSG), orother low-k or ultra low-k dielectrics may be used.

A metallic barrier layer 114 is located between the copper interconnect100 and the trench 108 and via 106 sidewalls. Barrier layer 114 preventscopper from diffusing into the ILD 110 and IMD 112. Barrier layer 114also provides adhesion between the copper and dielectric. Variousbarrier layers are known in the art. For example, refractory metals,refractory metal-nitrides, refractory metal-silicon-nitrides, orcombinations thereof may be used.

Copper interconnect 100 of the preferred embodiment includes Si dopant115. As discussed further below, doping with Si occurs after copper CMP.Accordingly, Si dopant 115 is gaseously introduced over the surface ofcopper interconnect 100. The concentration of the as deposited Si dopantis approximately 0.1% Si. As discussed above, doping the copperinterconnect 100 with Si improves the EM and VSM lifetimes.

A method of fabricating copper interconnect 100 according to theinvention will now be discussed with reference to FIGS. 3A-3E. Referringto FIG. 3A, semiconductor body 102 is processed through the formation ofone or more metal interconnect layers. ILD 110 and IMD 112 are depositedover semiconductor body 102. Suitable materials, such as FSG or OSG, forILD 110 and IMD 112 are known in the art. Trenches 108 are formed in IMD112 and vias 106 are formed in ILD 110, using conventional processing.

Barrier layer 114 is formed over IMD 112 including within trenches 108and vias 106. Barrier layer 114 functions as a diffusion barrier toprevent copper diffusion and as an adhesion layer. Transition metals andtheir nitrides are typically used for barriers. A transitionmetal-silicon nitride as well as combinations of transition metals,transition metal-nitrides and transition metal-silicon-nitrides may alsobe used. As an example, barrier layer 114 may comprise a Ta—N basedsputtered film.

Still referring to FIG. 3A, a copper seed layer 116 may be depositedover barrier layer 114. Physical vapor deposition is traditionally usedto form copper seed layer 116. Other methods for forming copper seedlayer 116, such as CVD are known in the art. Copper seed layer 116 ispreferably undoped. The copper seed layer 116 is needed to pass currentand to serve as a nucleation layer for the copper ECD process.

After deposition of the copper seed layer 116, the wafer is transferredto the ECD tool. Copper film 118 is formed by ECD. Various suitable ECDprocess are known in the art. In a preferred embodiment of theinvention, the wafer is transferred to the plating cell of an ECD tooland a plating process is conducted. The plating step may include severalplating currents as is well known in the art. The resulting copper film118 is shown in FIG. 3B. While ECD copper is described herein, theinvention is not limited to ECD deposited copper.

After the final copper thickness is achieved, the copper film 118 may beannealed. After the ECD process and any annealing steps, the copperlayer 118 (which incorporates seed layer 116) and barrier layer 114 arechemically-mechanically polished to remove the excess material above asurface level of IMD 112. The remaining copper and barrier 114 in thetrench forms copper interconnect 100, as shown in FIG. 3C.

After the copper CMP and prior to depositing any subsequent layers, suchas the etchstop layer for the next metal interconnect level, the surfaceof copper interconnect 100 is doped with silicon, as shown in FIG. 3D.In the preferred embodiment of the invention, silicon doping is achievedin the same chamber used for depositing the subsequent etchstop layerafter CMP, any post CMP cleans and pre-etchstop deposition cleans areperformed. Prior to striking a plasma in the chamber to begin SiNdeposition, the semiconductor body is heated to approximately 425° C.and SiH₄ (silane) gas is allowed to flow into the chamber and contactthe surface of copper interconnect 100. Allowing the silane gas to flowfor a few (0.5 to 5) seconds prior to striking the plasma (i.e., withthe RF power still off), dopes the surface of the copper interconnectwith silicon. For example, flowing the silane gas at a rate of 670 sccmsfor approximately three seconds prior to striking the plasma resultsultimately in a silicon doping of approximately 0.1% for trench depthsof ˜0.45 microns. Silicon doping in the range of 0.03 at. % to 0.5 at. %is desired. Various methods for doping the surface of copperinterconnect 100 will be apparent to those of ordinary skill in the arthaving reference to this specification.

It should be noted that the doping levels of the instant invention donot result in silicidation at the copper surface. At ˜400° C., the solidsolubility limit of Si in pure Cu is ˜8 at. %. This is well above thelevels of the instant invention. The duration of the exposure to SiH₄ isshort (on the order of 3 sec.) in the instant invention, whereas normalsilicidation exposure times are much longer (e.g., greater than 5 min.).

It should also be noted that Si doping negatively impacts resistivity ofthe copper interconnect. The resistivity increase depends on the finaldoping concentration which, in turn, depends on film thickness.

After doping the surface of the copper interconnect 100 to the desiredlevel, a plasma is struck in the chamber and a SiN etchstop layer 120 isdeposited as shown in FIG. 3E. As illustrated in FIG. 6 and discussedabove, silicon doping of the copper interconnect improves SiN adhesionto the copper surface. Processing may then continue to form the nextmetal interconnect level (i.e., a second ILD, IMD, trench, vias,barrier, and copper), any additional metal interconnect levels and topackage the device.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

1. A method for fabricating an integrated circuit, comprising the stepsof: forming a dielectric layer; forming openings in the dielectriclayer; filling said openings with a barrier, a copper seed, and anelectroplated copper film; chemically-mechanically polishing said copperfilm; and after chemically-mechanically polishing said copper film,forming a silicon nitride layer by: transferring the semiconductor bodyto a chamber; gaseously doping the copper film with silicon withoutforming a copper silicide by flowing a gas chemistry consisting ofsilane over the copper film in the chamber with the RF power off priorto striking a plasma; striking said plasma in said chamber after flowingsaid silane for at least 0.5 seconds; and then flowing at least onenitrogen-containing source gas into said chamber to deposit a siliconnitride layer over said copper interconnect.
 2. The method of claim 1,wherein said doping step dopes only a top region of the copper film withsilicon.
 3. The method of claim 1, wherein said doping step dopes asurface of said copper film that leads to a final bulk siliconconcentration in the range of 0.03 at. % to 0.5 at. %.
 4. The method ofclaim 1, wherein said dielectric layer comprises an interleveldielectric and an intrametal dielectric.
 5. The method of claim 4,wherein said openings comprise vias in the interlevel dielectric andtrenches in the intrametal dielectric.
 6. The method of claim 1, whereinsaid doping step comprises flowing silane over the copper film for aduration in the range of 0.5 to 5 seconds at 325° C.-425° C.
 7. A methodof fabricating an integrated circuit, comprising the steps of: providinga semiconductor body having a trench formed in a dielectric layer at asurface thereof; forming a copper film over the semiconductor bodyincluding with said trench, chemically-mechanically polishing the copperfilm to form a copper interconnect; after said chemical-mechanicalpolish step, doping said copper interconnect with silicon withoutforming a silicide by flowing silane over a surface of the copperinterconnect with an RF power off prior to striking a plasma; whereinsaid doping step comprises part of a silicon nitride derosition process;and wherein said silicon nitride deposition process comprises the stepsof: transferring the semiconductor body to a chamber; performing saiddoping step by flowing silane in said chamber for a given time priorstriking said plasma in said chamber; striking said plasma in saidchamber after flowing said silane for at least 0.5 seconds; and thenflowing at least one nitrogen-containing source gas into said chamber todeposit a silicon nitride layer over said copper interconnect.
 8. Themethod of claim 7, wherein said silane is flowed over the surface of thecopper interconnect for a duration of approximately 3 seconds.
 9. Themethod of claim 7, wherein said silane is flowed over the surface of thecopper interconnect for a duration in the range of 0.05 to 5 seconds at325° C. to 425° C.